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A previously introduced method by which prehepatic beta-cell secretion is calculated in vivo from plasma measurements of insulin and C-peptide was applied to data derived from iv glucose tolerance tests performed in normal women. Prehepatic secretory rates calculated using the combined model appeared biphasic in nature after glucose injection. Basal insulin secretion was 63.9 +/- 9.8 pmol/min. The duration of first phase was approximately 5 min, with secretion reaching a peak of 2033 +/- 342 pmol/min. The first phase was followed by a significant refractory period in which the secretory rate fell below basal values. The magnitude of second phase secretion was small relative to first phase secretion and appeared pulsatile in nature. Total integrated insulin secretion was 22.2 +/- 2.7 nmol, of which first phase accounted for 32%, and second phase accounted for the remaining 68%. Total incremental integrated secretion was 10.6 +/- 1.4 nmol, accounting for approximately half of the total insulin secretion. Proportions of first and second phase secretion changed to 66.5% and 33.5%, respectively, with incremental data. This study shows that the combined model of insulin and C-peptide is capable of estimating prehepatic insulin secretion from the iv glucose tolerance test and may provide a useful tool to measure secretion in vivo under various pathological conditions.